In previous film strip inspection systems, typically for 16 mm film and which are occasionally adaptable to 8 mm or 35 mm motion picture film, various techniques have been proposed for sensing defective sprocket holes, improper thickness variations in the film such as produced by improper splicing techniques, and cracks or cuts in edges of the film.
For example, in U.S. Pat. No. 3,759,095, feelers are provided such as sapphire beads or the like attached to spring arms. The feelers ride along the surface of the film and capacitance changes are measured as a result of a movement of the feeler arm. Additionally, a spring mounted roller in contact with the film can sense for thickness changes which are translated into capacitance changes. Another feeler system is disclosed in U.S. Pat. No. 3,935,468 wherein the feeler is attached to a light blocking "flag" wherein movements of the feeler arm are translated into varying amounts of light received.
U.S. Pat. No. 4,166,700 discloses the use of a beam of light directed tangent to a surface of the film at a roller wherein thickness variations of the film are sensing as changing variations in light intensity received. In this same unit, it is also necessary to provide feeler arms which ride along edges of the film to sense for cracks or cuts and also sprocket hole defects.
U.S. Pat. No. 4,264,825 is a system for determining an abnormal length of a film splice and employs two light beams spaced at a predetermined distance, a thickness variation in the film changing the amount of light received by blocking out additional light. The system disclosed only measures for abnormal splice length.
Finally, U.S. Pat. No. 4,276,547 discloses the use of three separate feeler systems which ride on a surface of the film for respectively sensing cuts at a sound track edge of the film, abnormal length sprocket holes, and abnormal film thickness variations. Capacitance phase change sensing is employed with each of the above-identified feeler arms.
In the prior art discussed above and other similar systems in this field, in a film inspection system which analyzes not only the thickness but also edge cuts and sprocket hole damage, at least one feeler system is employed and/or with at least two separate sensing locations. Thus, when running a motion picture film through the film inspection station, the station can impart some wear to the film in view of the contact of feeler arms onto a surface of the film. Also, the plurality of stations involved increases complexity of the mechanical arrangement and increases the difficulty of threading the motion picture film into and through the film inspection station.
Also, in the prior art systems, such systems can be unduly sensitive and require frequent alignment as a result of dirt, dust, or other contaminants occurring on feeler arms, light sources, light receivers, and other related contaminations or parameter variations resulting in signal variations.
Also, in such prior art systems, it can occur that sprocket holes are not reliably inspected for defects because of insensitivity due to creases, dirt, and edge lettering on the sprocket hole edge of the film.
Thus, in the prior art systems, a number of variables such as light intensity, film reflectivity, light transmission of optics, sensitivity of photocells, dirt on the lens, a whitening of a black contrasted area can all result in varying signal levels which make it difficult to reliably detect defects given the passage of time and use of the film inspection station.
A variety of types of film must be accommodated such as white, black, shiny, and colored film. Also, sensing for an absence of material, such as in the case of a sprocket hole, thus has caused problems in prior art systems.
Additionally, in prior art systems static charges which accumulate at the film inspection station have been known to affect detection circuitry and thus result in failures or inaccurate defect or flaw detection.